1. Field of the Invention
The present invention relates to a cooling fan for use inside a circuit system, in particular, a pulse-width modulation speed controllable DC cooling fan which can automatically vary its speed according to the internal temperature of the circuit system. While the internal temperature of the circuit system rising, the thermistor in the pulse width modulation fan speed control circuit will decrease the resistance and increase the current to increase the rotation speed of the fan such that the internal air inside the circuit system is forced to flow fast and carries away the heat emitted inside the circuit system resulting in a normal function of the system being kept. On the other hand, while the temperature is falling, said speed control circuit will cut down the power supply by increasing the resistance of the thermistor to reduce the rotation speed of the fan and the undesirable noise. Additionally, the control IC is a low power consumption complement metal-oxide-semiconductor (CMOS) so that it is extremely energy saving.
2. Description of the prior art
Circuit systems have wide range of application in various mechanical devices and control apparatus and bring a various convenient modern life for human beings. However, a circuit system must exert functions through the conductance of a current, so that unavoidably, heat will produced by the energy flow. If the heat can not be deprived the circuit system will be made to operation abnormally or be damaged. The most common type of conventional speed controllable cooling fan is an usual fan driven by a speed control circuit of voltage drop type which is provided with an expensive high power thermistor positioned between the fan motor and the power supply for the purpose of controlling the rotation speed of the fan motor by the variation of the voltage and current of the input power by means of altering the resistance of the thermistor as the temperature changes. But once the power consumption of the fan motor is too high or the voltage of the power supply is too low, the high power thermistor will not function normally and on the contrary, it will emit much heat to even increase the internal temperature of the circuit system.
Still further, a voltage regulator type speed control circuit has also been used which has a voltage regulator and a conventional thermistor inserted in between the power source and the fan motor and additionally has a power supply circuit provided at the beginning for designing the circuit system thereof. In addition to occupying more space of the circuit system, upon consideration of low voltage driving of the fan motor, a low voltage operatable driving IC must be chosen for the above two fan motors. Better design for the motor even bothers the choice of fan housing material and the manufacturing process. Besides, the speed control circuit results in heat producing problems of the circuit system.